1. Field of the Invention
The present invention relates generally to machining fluids and a method for machining using a machining fluid.
2. Description of the Prior Art
The intricate characteristics of hardness, toughness, wear, and corrosion resistance of advanced materials such as silicon nitrides have made them ideal candidates for high-performance structural applications. These unique properties, on the other hand, also have made them very difficult to machine. For example, their high hardness makes it necessary to use diamond tools to shape and fabricate them, and if the machining force is too high, surface cracks can render the materials unusable. Therefore, the current machining technology uses very shallow penetration depth and slow machining rate to reduce the potential of surface damage. This makes the machining process long and expensive. Tool wear of the diamond also adds to the cost. The G ratios (amount of diamond wheel wear to amount of material removed) are typically small when compared to the case of conventional machining of metals by machine tools. Accordingly, the cost of machining advanced materials has become very high, amounting to 50 to 90% of the component cost of tough materials such as silicon nitrides.
Various compounds have been employed to assist the machining of ceramics, but none of them is entirely satisfactory. For example, it has been reported that barium carbonate powders can be used to provide improved polishing of ceramics (See Yasunaga et al., "Mechanism and Application of the Mechanochemical Polishing Using Soft Powders," NBS SP 561 (1979)). Improved polishing of quartz using Fe.sub.3 O.sub.4 and MgO has also been described. Vora et al. have described the use of Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 for polishing silicon nitride in "Mechanochemical Polishing of Silicon Nitride," Am. Cer. Soc., p. C140 (1982), and the polishing of boron carbide by NiO or SiO.sub.2 is described in "A Study of Mechanochemical Machining of Ceramics and the Effect on Thin Film Behavior," Office of Naval Res., No. N00014-80-C-0437-2 (1983). All of these processes suggest that by using their powders, some thin films on the order of 1000 .ANG. thick are formed on the ceramic surface, and that these films can be easily removed without causing damage to the surface. These processes result in a reduction of residual surface damage and improved surface quality, but no increase in the material removal rate. In fact, many of these actually decreased the material removal rate and none of these processes provided an improved machining rate over the conventional diamond tool methods.
In attempting to modify the ceramic surface using chemicals, the reactivity of compounds with ceramics must be considered. At first, it might seem that chemicals that work for metals might work for ceramics, but there are fundamental differences in the chemical reactivity between these material classes. In general, ceramics are considered to be chemically inert, while metals are relatively reactive. This occurs because the nature of interatomic bonding is different within metals and ceramics. Metals are held together with metallic bonds, and the electrons are free to move about in the metal. Ceramics are held together by covalent or ionic bonds. Because of the nature of the bonding, ceramics are inherently inert until the bonds are disrupted. Gates in his Ph. D. dissertation (Penn State University, December 1993) has suggested that the dangling bonds of silicon are responsible for the tribochemical reactions taking place between rubbing silicon nitride surfaces. He further found that common antiwear compounds which are effective for metal are not effective in lubricating ceramics. In metals, organometallic chemistry dominates, and in ceramics, surface hydrolysis reactions dominate. Therefore, in examining different chemistries for ceramic machining, the teachings of previous patents in metal-working fluids are not applicable.
Previously, using a diamond blade cutting machine, Wang and Hsu (Journal of Tribology, July 1994, Vol. 116, pp. 423-429) surveyed many chemicals for silicon nitride machining. They discovered, as described in U.S. Pat. No. 5,447,466, that polyhalogenated hydrocarbons were effective in accelerating the cutting rate and at the same time, improving the surface finish of ceramics.
Even the general subject of machining ceramics is relatively new, and the physical and chemical principles involved are not well understood. The current state of the art has been summarized by S. Jahanir et al. in their survey, Ceramic Machining: Assessment of Current Practice and Research Needs in the United States (NIST Special Publication 834, May 1992), and also in Cost Effective Ceramic Machining Highlights (ORNL Report ORNL/M-2827, Apr. 30, 1993). These reports essentially state that many of the basic phenomena in ceramic machining are not understood and that current practice is primarily based on experience and trial and error. Optimization of the wheel, load, speed and the materials to be machined has not been done. The effects of the coolant (machining fluid) used during machining are also not understood.